1. Field of the Invention
This invention relates to a sheet discharging apparatus included in an image forming apparatus or a sheet post-processing apparatus connected with the image forming apparatus.
2. Discussion of the Background
In a background sheet post-processing apparatus, for example, as illustrated in FIG. 23, a sheet discharging device is provided at a discharging exit, which includes a sheet discharging roller 200 as a driving roller, and a driven roller 202 which can contact and separate from the sheet discharging roller 200. The sheet discharging device is configured to discharge a sheet or a set of stapled sheets on a sheet discharging tray 206 with the sheet or the set of stapled sheets sandwiched and conveyed between the sheet discharging roller 200 and the driven roller 202.
The height of the sheets stacked on the sheet discharging tray 206 is detected by a sheet surface detecting device which includes a sheet surface detecting lever 208 positioned at substantially a center of a sheet discharging tray 206, and sensors 210 and 212. When the height of the sheets stacked on the sheet discharging tray 206 reaches a predetermined height, the sheet discharging tray 206 is driven to move down so as to receive additional discharging sheets. If the sheet discharging tray 206 is fixed to the sheet post-processing apparatus, the above-described sheet surface detecting device functions as a full stacked sheets detecting device which detects that the sheets are stacked to the full capacity on the sheet discharging tray 206.
Referring to FIG. 23, a reference numeral 214 designates a guide plate which guides the sheet or the set of stapled sheets to the discharging exit of the sheet post-processing apparatus, a reference numeral 216 designates a discharging sensor, and a reference numeral 218 designates a pullback roller which pulls back the discharged sheet to an end fence (not shown) of the sheet discharging tray 206 so as to align the trailing edge of the discharged sheet.
As illustrated in FIG. 23, when the set of stapled sheets 204 is discharged on the sheet discharging tray 206, the stapled sheets 204 are prone to have so-called back curls, i.e., the tip portion of the stapled sheets 204 is curled downward by gravity depending on thickness, weight, or material of the stapled sheets 204. The back curl increases an angle (.alpha.) formed between the stapled sheets 204 and the surface of the sheet discharging tray 206 at the contact point of the tip portion of the stapled sheets 204 with the surface of the sheet discharging tray 206.
If the discharging operation of the set of stapled sheets 204 continues in the above back curl condition, the stapled sheets 204 may be stacked on the sheet discharging tray 206 with its tip portion bent inward, i.e., the tip portion of the stapled sheets 204 U-shaped, as illustrated in FIG. 24. The above-described bend of the stapled sheets 204 may cause inadequate stacking and a detection error of the sheet surface detecting device.
The above-described sheet tip portion bending problem occurs not only in the discharged set of stapled sheets but also in the sheet which is discharged one by one on the sheet discharging tray 206.
In order to address the above-described sheet tip portion bending problem, as illustrated in FIG. 25, a known sheet discharging apparatus includes a protrusive member 220 on the surface of the sheet discharging tray 206 such that a relatively small angle is formed between the tip portion of a sheet or a stapled sheets 204 and the surface of the protrusive member 220. Thereby, bending of the sheet or the set of stapled sheets 204 is avoided. The protrusive member 220 is made detachable from the sheet discharging tray 206.
However, in the above-described sheet discharging apparatus with the protrusive member 220, when a sheet is discharged on the sheet discharging tray 206 one by one (hereinafter called a shift stack mode), the surface of a trailing end part of the stacked sheets is projected due to the protrusive member 220 as illustrated in FIG. 26. In addition, the trailing edge of some sheets does not reach a position to contact a surface of the pullback roller 218 so as to be pulled down by the pullback roller 218 to be abutted against the end fence. As a result, some of the sheets cannot be pulled back surely to be abutted against the end fence, and thereby inadequate stacking is caused.
Moreover, because the surface of the stacked sheets is projected on the protrusive member 220, as illustrated in FIG. 27, a sheet stacking on the stacked sheets does not slide on the surface of the stacked sheets to the end fence by gravity after the trailing edge of the sheet falls down to the surface of the stacked sheet. As a result, the trailing edge of the sheet does not reach a position to contact a surface of the pullback roller 218 so as to be pulled down by the pullback roller 218 to be abutted against the end fence. Thereby, the sheet is not pulled back to be abutted against the end fence, and the sheet is stacked with its leading edge positioned forward.
Further, in the shift stack mode, when the sheet has so-called face curls at the trailing end portions thereof in the sheet discharging direction (i.e., the trailing end portions of the sheet are curled upward), and when the sheet is discharged on the protrusive member 220, the trailing edge of the sheet is positioned above the surface of the sheet discharging tray 206. As a result, the trailing edge of the sheet contacts the sheet discharging roller 200 and is pulled by the sheet discharging roller 200.
In light of the above-described inadequate sheet stacking, when the sheet is discharged in the shift stack mode or the sheet is prone to have face curls, the protrusive member 220 is detached from the sheet discharging tray 206 in the sheet discharging apparatus.
As stated above, in the background sheet discharging apparatus with the protrusive member 220, an operator is inconvenienced by having to detach and attach the protrusive member 220 according to the types of sheets and stacking mode. Further, when the protrusive member 220 is detached from the sheet discharging tray 206, the advantage of preventing the sheet tip portion bending problem may be abandoned.
In Japanese Laid-open Patent Publication No. 6-329320 of 1994, an image recording apparatus is described in which a recording condition of a sheet can be visually confirmed one by one in a sheet facedown discharging method.
As illustrated in FIG. 28, the characteristic of the above image recording apparatus is that a hook member 224 including a latch part 224a at its tip portion, is swingably provided in the vicinity of a downstream side of the sheet discharging device 222. The hook member 224 swings upward by being pushed by a recorded sheet P, and the sheet P is directed in the sheet discharging direction with the leading edge of the sheet P caught by the latch part 224a of the hook member 224. As the tip portion of the sheet P is turned up by being caught by the latch part 224a toward the front side, the recording condition on the underside of the sheet P can be confirmed visually. As the sheet P advances in the sheet discharging direction, the hook member 224 is pushed to swing upward, and then the leading edge of the sheet P is released from the latch part 224a and the sheet P falls down to be stacked.
In the above-described image recording apparatus of JP No. 6-329320, the sheet tip portion bending problem can be avoided because the tip portion of the sheet is turned up. However, when a sheet having a strong tension, such as a thick paper or a set of stapled sheets is discharged, the sheet or the set of stapled sheets configured to stretch between the latch part 224a and a discharging exit of the image recording apparatus due to the strong tension and the leading edge of the above sheet or sheets may not be released from the latch part 224a in an appropriate manner. As a result, inadequate discharging of the sheet or the sheets may be caused.
In another Japanese Utility Model Publication No. 7-45498, a sheet discharging apparatus is described in which a tip portion of a sheet is pinched by a claw on a chain and the sheet is transferred by driving the chain. The sheet is released from the claw at a predetermined position with a cam mechanism.
In the above-described sheet discharging apparatus of JP No. 7-45498, the sheet is discharged adequately without being bent, because the sheet is released from the claw at a designated position. However, a chain driving mechanism, and the cam mechanism which regulates the sheet release from the claw, increase the cost of the sheet discharging apparatus. In addition, because the sheet is transferred with the leading edge of the sheet pinched by the claw, a pinching mark remains at the leading edge of the sheet.